The present invention relates to devices for and/or comprised in antennas, in particular microwave antennas, with which control and modification of the antenna lobe can be carried out. The invention also relates to devices for and/or comprised in antennas, in particular microwave antennas, with which coupling between the polarizations in dual-polarized radiating antenna elements can be reduced or completely avoided. This application is based on application No 9700536-7 filed in Sweden, the content of which is incorporated hereinto by reference.
The development of mobile telephony has created a need for simple and inexpensive mass production of antennas for, inter alia, base stations. Mobile telephony utilizes freqency ranges which lie within the microwave range, for which reason a type of antenna which is often used is microstrip antennas. Microstrip antennas consist of radiating antenna elements which are mounted in front of a ground plane. A base station usually uses a number of antennas in order to cover a cell. Different mobile telephony operators make use of different algorithms for cell planning, which results in a demand for antennas with, inter alia, different lobe widths. One known way of controlling the lobe width of a microstrip antenna is to modify the dimensions of the ground plane. If a large lobe width is desired, the size of the ground plane is limited. A major disadvantage of the size of the ground plane being reduced in the case of a large lobe width being desired is that this also limits the possibility of, for example, using a microstrip distribution network as a feeder network for the radiating elements, Therefore, antennas with wide lobes cannot on the whole use microstrip distribution networks as feeder networks or they can use them only to an extremely limited extent. If desired, antennas with narrow lobes can in most cases use microstrip distribution networks as feeder networks, which is advantageous as far as, inter alia, manufacturing aspects and therefore the costs of these antennas. Another major disadvantage of modifying the size of the ground plane in order to control the lobe width is that the shape and size of the antenna are influenced, that is to say that different antennas must be designed and subsequently manufactured for different desired lobe widths. Modifying the size and shape of an antenna results in a number of consequential problems such as, for example, the need for different weather protection (radomes) and modified mounting arrangements.
In order to achieve better and more reliable coverage, which is of particular importance within mobile telephony, use is made of polarization diversity. Antennas, in particular microwave antennas, advantageously use dual-polarized radiation elements as this reduces the size and production costs of the antenna compared with the use of single-polarized radiation elements. A requirement has arisen, in particular within mobile telephony, for antennas with .+-.45.degree. polarization because this type of polarization has proved to have many advantages, such as more symmetrical propagation/attenuation, in relation to 0/90.degree. polarization Unfortunately, it has proved to be difficult, in relation to 0/90.degree. polarization antennas, to manufacture .+-.45.degree. polarization antennas, and in particular microstrip antennas, with dual-polarized radiation elements which have satisfactory isolation between the polarizations, that is to say low cross-coupling.